2002/04/12 by V. Brouet, H. Alloul, S. Garaj +3 · 2 citations
Chemistry · Materials Science · Physics and Astronomy · #Boron and Carbon Nanomaterials Research #Diamond and Carbon-based Materials Research #Fullerene Chemistry and Applications #cond-mat.str-el
paper · pdf · doi:10.1103/physrevb.66.155122
12 pages, 13 figures
arxiv created 2002/04/12 · openalex publication_date 2002/10/31 · arxiv updated 2009/11/30 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
We present a nuclear magnetic resonance (NMR) study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^2\ensuremath- or C60^4\ensuremath-. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows one to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected by the appearance of quadrupole effects on the 23Na spectra of Na2C60. In K4C60, modifications of the 13C spectra line shapes also indicate a structural modification. We discuss this high-temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T1T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.